This application relates to a motor bus voltage compensation method in a system wherein a motor drives a shaft against a mechanical load force.
Motor drive systems are known, wherein a control sends a signal to a switching network, or “drive bridge”, that converts the supplied DC power to three-phase regulated power to drive a motor. A control section regulates power through and out of the switching network to regulate motor power and direction control. The control uses a position signal from a sensor on the motor shaft to determine the appropriate combination of signals and timing to the switching network to create the desired motion, otherwise known as the motor commutation. Often, the control will add a positive angular offset to a commutation signal to provide additional torque.
In some applications, the motor may drive a shaft in a first direction when actuated, while a mechanical bias, such as a spring, will return the shaft when the motor is deactivated. The motor may or may not be actively driven in the bias direction, depending on acceleration and velocity requirements.
The motor bus supplies power to the switching network. The motor bus power is supplied through an input power conditioner section. The input power conditioner may include a reverse current blocking diode to prevent power surges on the external power bus. In this case the power conditioner will also have power storage, such as a capacitor, to store power to and from the motor. A power conditioner may also contain rectifier circuitry to convert input power from AC to DC, and/or voltage regulation such as a buck or boost converter to control the motor bus voltage.
When the shaft is being driven in the reverse direction by the mechanical bias, reverse current flow can be generated from the motor back into the motor bus. This reverse current flow reaches the switching network and power conditioning section. The voltage spike provided by this reverse current flow can raise concerns at the switching network and power conditioning section. These concerns and the magnitude of the voltage spike are increased if the positive offset is still being added to the commutation signal.
It is more costly and requires more circuit board space to provide extra capacitance or an active voltage clamp to address this voltage spike.